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Hypothetic Question: is is possible combine PW2 and DIY storage

solarAddict

Member
Jan 26, 2020
102
42
MA
Friday afternoon topic.

Not that I'm planning (or capable) to do so but curious mind want to know if it possible to build a DIY battery and hook it up to existing system in order to increase capacity. Note, I'm not talking about legal side of it as well, but rather technical.
 

jjrandorin

Moderator, Model 3, Tesla Energy Forums
Nov 28, 2018
7,681
8,460
Riverside Co. CA
Not that I can answer your question, but I think what you mean by "hook it up to existing system" would dictate whether what you are asking is possible. "hook up to existing powerwalls and have the TEG gateway control everything"? Likely no.

"Hook up to existing system" meaning taking loads that may be in a non backed up panel, and backing them up with a DIY system that is in no way connected to the tesla teg + powerwalls? I am sure that could be done.

I doubt there is any way to hook up any sort of DIY system in such a way that it still interfaces with / is visible to the tesla system, but I dont see why one could not setup a separate system, just like its possible to have 2 completely separate PV systems on ones home that are not connected to each other.
 

solarAddict

Member
Jan 26, 2020
102
42
MA
When I mean 'hookup to existing system" I meant the first variant you listed. The system with PW2 is getting additional capacity by adding DIY storage, perhaps not visible in phone app but still included in total capacity by the invertor...
 

BGbreeder

Member
Jun 19, 2020
139
76
Bay Area
Ok: no personal knowledge, but a DIY battery added to a Powerwall system would have to navigate the negotiation process of turning down / off the solar based on the state of charge for the DIY battery and for the state of charge for the powerwalls and get each subsystem to behave properly when they are in discordant states. Pushing / pulling from the grid is easy. Dealing with frequency shifting not so much. What device would be the controller, and how would it get input from different devices with different power characteristics?

I'm not saying it is impossible, but I don't think that it is trivial either. Presumably there is some smarts somewhere in the existing Powerwall system(s) to handle different aging rates of different powerwalls to align charge/discharge curves, but I have never seen a reference. If you insert another source/load (your DIY battery) the gateway is going to need to be able to account for it correctly and vice versa.

Rather like the old problem of getting tandem horses or oxen to pull at the same effort. That wasn't easy and it got harder the more horses you added.

All the best,

BG
 

jhn_

Member
Jan 21, 2021
85
63
Northeast United States
The DIY battery would have to be wired into the backup sub panel. The TEG and PWs would have no idea it is there it would just look like negative house load. This is the way my generator is hooked up.
 

miimura

Well-Known Member
Aug 21, 2013
6,074
5,657
Los Altos, CA
A DIY battery system could be integrated with a Powerwall system in different ways.

1. Small grid tied inverters could be used to offset household use. If the output was significantly larger than the household use, then it could cause interoperability problems if the Powerwall system didn't treat it like solar.
2. A large off-grid inverter could be used during grid outage situations to act like the grid. In this case you would have to be careful about overload conditions on the DIY side because it would have to handle all household loads once the Powerwalls got down to their Reserve level. You would also have to be careful not to back-feed the grid because you would necessarily have to connect it on the grid side of the Tesla Gateway.

There are surely more use cases. These are only theoretical. I have done some tests described here: Powerwall 2.0 Backup Runtime Extender
 

BGbreeder

Member
Jun 19, 2020
139
76
Bay Area
The DIY battery would have to be wired into the backup sub panel. The TEG and PWs would have no idea it is there it would just look like negative house load. This is the way my generator is hooked up.
So, how do the Powerwalls deal with the generator and solar as they approach full charge? Or do you preemptively shut the generator down as the charge level gets high.

Just curious.

All the best,

BG
 

jhn_

Member
Jan 21, 2021
85
63
Northeast United States
So, how do the Powerwalls deal with the generator and solar as they approach full charge? Or do you preemptively shut the generator down as the charge level gets high.

Just curious.

All the best,

BG
I haven’t tested all scenarios, but the in “normal” use the generator is tied to a transfer switch that senses when grid power is down. Before the PW were installed that was set to auto, now I have it set to manual.

I have tested when the batteries were off, near empty, and charging from solar. The generator adjusts output and the batteries don’t do much in those tests because the generator meets the load of the house. I don’t really see running in this scenario, the generator was here before the PWs and I would only use it if we were off grid and the PV/PW didn’t have enough such as a winter multi-day storm outage. We are in an area that gets serious winter storms.

I guess the answer to your question is I haven’t tested the scenario with PWs near full, but I don’t see that being a use case where I would manually turn the generator transfer switch on.

So the OP could hook up DIY batteries through a manual transfer switch to backup panel of a PW install. Wouldn’t be hands off and seamless, but it would work.
 
Last edited:

CrazyRabbit

Member
Apr 21, 2020
340
105
Fort Worth TX
The answer is no. To work in tandem, the diy system would have to communicate with the gateway which is a closed api.
perhaps sol-ark would be better solution where you can build your own battery packs. You can stack 72Kw, which I think is six inverters.
 
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Reactions: solarAddict

jhn_

Member
Jan 21, 2021
85
63
Northeast United States
How do you synchronize the frequency when you turn the generator?

All the best,

BG
You don’t. Which is why there is a brief outage.

And per the other responses in the thread, I think two ideas are getting conflated here. One is having DIY batteries work seamlessly with the TEG, and the second which is having a DIY source of electricity physically wired into the same house as a TEG.

I’m only saying the latter is possible as we’ve done it. I’ll leave the former up to others to debate as it seems more of a long shot.
 

miimura

Well-Known Member
Aug 21, 2013
6,074
5,657
Los Altos, CA
You don’t. Which is why there is a brief outage.

And per the other responses in the thread, I think two ideas are getting conflated here. One is having DIY batteries work seamlessly with the TEG, and the second which is having a DIY source of electricity physically wired into the same house as a TEG.

I’m only saying the latter is possible as we’ve done it. I’ll leave the former up to others to debate as it seems more of a long shot.
That's right. The officially sanctioned way Tesla integrates generators ensures that the batteries never see the generator waveform because the generator transfer switch isolates it.
 

h2ofun

Active Member
Aug 11, 2020
1,561
250
auburn, ca
That's right. The officially sanctioned way Tesla integrates generators ensures that the batteries never see the generator waveform because the generator transfer switch isolates it.
Yep, it is impossible for my generator to ever see things in front of it, meaning the GW and anything hanging off it.
 

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